diff -r d3c7ebdc71bb Doc/reference/datamodel.rst
--- a/Doc/reference/datamodel.rst	Mon Oct 15 16:57:37 2012 +0200
+++ b/Doc/reference/datamodel.rst	Tue Oct 16 00:56:33 2012 -0400
@@ -1802,7 +1802,10 @@
    Called to implement the built-in function :func:`len`.  Should return the length
    of the object, an integer ``>=`` 0.  Also, an object that doesn't define a
    :meth:`__bool__` method and whose :meth:`__len__` method returns zero is
-   considered to be false in a Boolean context.
+   considered to be false in a Boolean context. A dynamic object with a
+   :meth:`__len__` method should report accurately or not at all raising a
+   :exc:`TypeError`. For more details on :meth:`__len__` see the docstring 
+   in the test file test_iterlen.py.  
 
 
 .. method:: object.__length_hint__(self)